1. Field of the Invention
The present invention relates to the protection of sensitive equipment from damaging vibrations created by external forces. More particularly, the present invention relates to a vibration-isolating apparatus for transporting equipment during subsea drilling operations.
2. Description of the Prior Art
The expanding search for oil and gas has extended drilling operations from shallow coastal regions into deeper water. As the depth of the water increases, the ability to monitor or directly control submerged equipment becomes more arduous. Divers are frequently used in shallow drilling operations, yet the problems of decompression and the deficiencies of isobaric diving may limit efficient diving operations to depths of 600 feet. While the range of conventional diving operations is limited to shallow water, subsea drilling operations frequently extend to depths greater than 1000 feet.
At greater drilling depths, many drilling and production operations are controlled remotely. Underwater television cameras and other equipment are lowered from a drillship or platform to perform a variety of services. Television cameras are routinely used to observe the landing of subsea equipment, cementing, and tool operations. Ultrasonic equipment is utilized in nondestructive testing to identify corrosion and other deficiencies in submerged equipment. The remote control of similar operations has proven indispensable in directing efficient subsea drilling operations.
Various guidance systems have been developed to transport equipment to the ocean floor. Guidelines attached between the sea floor and a drillship or platform are commonly used to guide equipment to a subsea wellhead. U.S. Pat. No. 3,184,541 illustrates a television camera deployed along a system of four guidelines anchored by a weighted guidebase to the ocean floor.
The use of guidelines is economical and reliable for drilling depths approaching 1800 to 2000 feet. Yet guidelines can prove impractical at greater drilling depths because of the increased size of the guidelines and guideline handling equipment required to manipulate a larger guideline system. Large guideline systems are not only expensive and unwieldly, but at greater depths the guidelines and drillpipe may become tangled due to ocean currents and vessel drift.
Consequently, guidelineless drilling systems have been developed for drilling in ocean depths greater than 2000 feet. Guidelineless systems are particularly appropriate for a dynamically positioned drilling vessel because of vessel movement, within a few degrees of center, about the subsea well. In guidelineless drilling operations, equipment-carrying tools transport equipment along the exterior of the drillpipe or casing. The tools are commonly lowered along the drillpipe by a cable or line controlled from the surface.
The deployment of equipment along the drillpipe in a guidelineless system is satisfactory in a static subsea environment, but the presence of strong ocean currents may induce drillpipe vibrations which jeopardize vibration-sensitive equipment. In a current, the flow of water around the drillpipe can create opposing vortices which are alternately shed from each side of the drillpipe. This vortex shedding creates pulsating forces on the drillpipe transverse and parallel to the current. Under certain circumstances, particularly where these forces correspond with the natural frequencies of the drillpipe, the vortex-induced forcing frequencies can cause the drillpipe to vibrate turbulently. This effect is more pronounced at greater drilling depths because a long drillpipe is exposed to greater current forces and has many natural frequencies.
The forcing vibrations and corresponding movement of the drillpipe induced by current flow can destroy vibration-sensitive equipment deployed near the drillpipe. A need, therefore, exists for an apparatus that will isolate equipment from excessive vibrations produced in a dynamic environment. The apparatus must attenuate a wide range of vibrational frequencies impinging on the apparatus.